It’s quite simple.
Choose your price range (search Vinted & eBay for similar items). State a preferred sale price but allow for offers to come in lower, and remember that the buyer pays fees. So take into account that the asking price they see is higher than you put up
Work out a suitable package size and weight (taking into account packaging)
Register, check your profile details for postage methods (Vinted will offer any courier by default. Disable any that are unsuitable for you to drop off at. Have a good think about Royal Mail as Vinted have a habit of charging the minimum postage, and sellers end up paying the extra for a heavier item)
take photos, list it
Put as much info as possible / that you can be bothered
Await messages - note that in Vinted an offer is not an offer, if you accept the ‘offer’ they still may not buy it - you never have a sale until it’s been paid for
Consider eBay, it’s pretty much the same as Vinted but people tend to buy particular things on one or the other.
I used to put clothes on Vinted and misc on eBay, but now just eBay with my Vinted listings gradually going down
With postage on eBay the default ‘simple’ postage means the buyer can choose Royal Mail or Evri and get charged the standard rate of the size / weight you specify
But with ‘custom’ you set the options available